Numerous proposals are known from the prior art to provide olefin polymerization catalysts by combining a solid component comprising at least magnesium, titanium and chlorine with an activating organoaluminum compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor (Lewis base) in the solid component and by employing as a third catalyst component an electron donor which may be complexed in whole or in part with the activating organoaluminum compound.
For convenience of reference, the solid titanium-containing constituent of such catalysts is referred to herein as "procatalyst", the organoaluminum compound, whether used separately or partially or totally complexed with an electron donor, as "cocatalyst", and the electron donor compound, whether used separately or partially or totally complexed with the organoaluminum compound, as "selectivity control agent" (SCA).
Supported coordination catalyst of this type are disclosed in numerous patents. The catalyst systems of this type which have been disclosed in the prior art generally are able to produce olefin polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alpha-olefins, with high selectivity to stereoregular polymer. However, further improvements in productivity at high stereoregularity are still being sought.
The objective of workers in this art is to provide catalyst systems which exhibit sufficiently high activity to permit the production of polyolefins in such high yield as to obviate the necessity of extracting residual catalyst components in a deashing step. In the case of propylene and higher olefins, an equally important objective is to provide catalyst systems of sufficiently high selectivity toward isotactic or otherwise stereoregular products to obviate the necessity of extracting atactic polymer components.
Although many chemical combinations provide active catalyst systems, practical considerations have led the workers in the art to concentrate on certain preferred components. The procatalysts typically comprise magnesium chloride, titanium chloride, generally in tetravalent form, and as electron donor an aromatic ester such as ethyl benzoate or ethyl-p-toluate. The cocatalyst typically is an aluminum trialkyl such as aluminum triethyl or aluminum tri-isobutyl, often used at least partially complexed with selectivity control agent. The selectivity control agent typically is an aromatic ester such as ethyl-paramethoxybenzoate(ethyl anisate) or methyl-p-toluate.
While the selection of cocatalyst and selectivity control agent affects the performance of those catalyst systems, the component which appears to be subject to most significant improvement with respect to activity and productivity of the system is the procatalyst.
Preferred methods of preparing such procatalysts are claimed in U.S. Pat. Nos. 4,329,253; 4,393,182; 4,400,302; and 4,414,132. These procatalysts are highly active and stereospecific. The typical manner of preparing such procatalysts involves the reaction of the magnesium compound, titanium tetrachloride and electron donor in the presence of a halohydrocarbon. The resulting solid particles are then contacted with additional quantities of TiCl.sub.4 and are completed by washing off excess TiCl.sub.4 using light hydrocarbons (e.g., isooctane and isopentane) and drying. Various studies have shown that the resulting procatalyst may suffer from shelf-life problems when stored as a dry powder. Even though the catalysts are always stored under nitrogen, a catalyst which is aged as a dry powder invariably loses activity over time. The exact cause of the activity loss is not known, but the most likely cause is contamination by impurities (especially water). This shelf-life problem was overcome by storing and shipping the catalyst as a slurry in dry mineral oil. The mineral oil slurry method has given excellent results, but has been found to suffer two disadvantages compared to dry-powder shipment:
(i) slurry concentrations vary considerably causing inconsistencies in polymerization plant operation when switching catalyst drums; PA1 (ii) introducing the catalyst as a mineral oil slurry is not optimum for all processes. For example, in gas-phase operation a dry powder or a slurry in a volatile hydrocarbon might be preferable. PA1 (a) said porous solid component comprises titanium tetrachloride, magnesium chloride and electron donor, having a pore volume of between about 0.1 and about 1.0 cubic centimeters per gram (cc/gm); PA1 (b) said hydrocarbon mineral oil is a viscous, paraffinic/naphthenic hydrocarbon oil; and PA1 (c) said mineral oil is mixed with said porous solid component such that the mineral oil fills the pores of said solid component and the resulting mineral oil-modified solid component remains relatively dry and free-flowing at ambient conditions. PA1 (a) halogenating a magnesium compound of the formula MgR'R" wherein R' is an alkoxide or aryloxide group and R" is an alkoxide or aryloxide group or halogen, with a halide of tetravalent titanium in the presence of a halohydrocarbon and an electron donor, separating the solid reaction product from the reaction mixture, optionally washing it with an inert diluent to remove any unreacted material; PA1 (b) contacting the halogenated product with a tetravalent titanium halide and separating the resulting solid from the liquid medium; PA1 (c) contacting the resulting solid with an inert light hydrocarbon liquid to remove unreacted titanium compounds; PA1 (d) adding a certain amount of a hydrocarbon mineral oil to the mixture of light hydrocarbon liquid and solid component wherein said mineral oil is a viscous paraffinic/naphthenic hydrocarbon oil; and PA1 (e) removing the light hydrocarbon liquid from the resulting mixture, wherein the amount of hydrocarbon mineral oil adding in step (d) is greater than five (5) percent by weight, based on the combined weight of the solid component plus mineral oil, but is less than the amount whereby the solid component is no longer free-flowing. PA1 (i) the addition of about 10 to about 25 percent by weight of mineral oil to certain procatalyst components still leaves the catalyst as a dry, free-flowing powder; PA1 (ii) such modified catalysts have demonstrated vastly improved activities in propylene polymerization; PA1 (iii) when stored as "dry powder" at ambient temperature over long periods (greater than 50 days), these catalysts maintain their activity much better than dry catalysts which have not been modified with mineral oil.
In U.S. Pat. No. 4,290,915, the patentee dries the catalyst such that the remaining catalyst contains 1-25% by weight of the inert liquid light hydrocarbon (e.g. n-pentane, cyclohexane, benzene and the like) used in the catalyst preparation. However, the patentee still finds that the catalyst loses activity after storage for as little as two days. Further, there are problems in ascertaining and controlling the level of the inert liquid hydrocarbon in the catalyst.
A new procedure has been found to prepare these catalyst components that not only eliminates the activity loss problem associated with dry catalyst, but also does not have the various disadvantages of the slurry method for storing catalysts.